Digital or solid state lighting technologies, i.e. illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g. red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626, incorporated herein by reference. LED technology includes line voltage powered white lighting fixtures, such as the ESSENTIALWHITE series, available from Philips Color Kinetics. These fixtures may be dimmable using trailing edge dimmer technology, such as electric low voltage (ELV) type dimmers for 120 VAC line voltages.
Many lighting applications make use of dimmers. Conventional dimmers work well with incandescent (bulb and halogen) lamps. However, problems occur with other types of electronic lamps, including compact fluorescent lamp (CFL), low voltage halogen lamps using electronic transformers and solid state lighting (SSL) lamps, such as LEDs and OLEDs. Low voltage halogen lamps using electronic transformers, in particular, may be dimmed using special dimmers, such as ELV type dimmers or resistive-capacitive (RC) dimmers, which work adequately with loads that have a power factor correction (PFC) circuit at the input.
Conventional dimmers typically chop a portion of each waveform of the input mains voltage signal and pass the remainder of the waveform to the lighting fixture. A leading edge or forward-phase dimmer chops the leading edge of the voltage signal waveform. A trailing edge or reverse-phase dimmer chops the trailing edges of the voltage signal waveforms. Electronic loads, such as LED drivers, typically operate better with trailing edge dimmers.
Unlike incandescent and other resistive lighting devices which respond naturally without error to a chopped sine wave produced by a phase-cutting dimmer, LED and other solid state lighting loads may incur a number of problems when placed on such phase chopping dimmers, such as low end drop out, triac misfiring, minimum load issues, high end flicker, and large steps in light output. In addition, even when a phase chopping dimmer is set to its highest setting in order to minimize the amount of dimming, the phase chopping dimmer still does not allow the full input mains voltage signal waveform to the input of a power converter, configured to deliver DC power to the LED or other solid state lighting load corresponding to the input mains voltage.
For example, FIG. 1A depicts waveforms of a rectified input mains voltage received by a power converter when a dimmer is connected between the voltage mains and the power converter, where the dimmer is set at its highest setting. FIG. 1B depicts waveforms of the received rectified input mains voltage when the power converter is connected directly to the voltage mains, without a dimmer (indicated by an “X” through the adjacent dimmer switch). As indicated by FIGS. 1A and 1B, the root mean square (RMS) voltage at the input to the power converter is slightly lower with a dimmer, as compared to the directly connected power converter. In other words, the power converter in the dimmable lighting system runs in a fashion that delivers less power with less RMS input voltage. As a result, power delivered to the solid state lighting load, even with the dimmer at its highest (no dimming) setting, is slightly less than the power delivered to the solid state lighting load without a dimmer.